x²+9x+20=x²+5x+4x+20=x(x+5)+4(x+5)=(x+4)(x+5)
x² + 9x + 20 =
= x² + 5x + 4x + 20
= x(x + 4) + 4(x + 5)
= (x + 5)(x + 4)
